SLIDING MESH GEARBOX

Sliding mesh gearbox is the oldest and simplest form of gear box. In order to mesh gears on the splined main shaft with appropriate gears on the layshaft for obtaining different speeds, they are moved to the right or left. It derives its name from the fact that the meshing of the gears takes place by sliding of gears on each other.

A three speed sliding mesh gear-box is shown in Figure. Splines are provided on the main shaft. For meshing the pinions with the matching gears on the layshaft, the pinions are slided along the spline. When the main shaft is driven from the layshaft the gear reduction is provided by the first pair of gears which are always in mesh. They are usually known as constant mesh gears. For changing gear the clutch is depressed and the gear lever is moved till the selector pinion on the main shaft engages with its mating gear on the layshaft. The drive from the engine will be again transmitted through the gear-box when the clutch is released. To obtain three forward speeds, reverse and neutral, the relative position of the gears will be as below:

First gear: The largest gear on the main shaft is driven by the smallest gear or pinion on the layshaft. With corresponding increase torque, the speed reduction is quite high. When climbing and moving off steep hill, starting the vehicle from rest this gear is usually used.

Second gear: In this gear, there is less speed reduction and smaller torque increase.

Third or top gear: In order to revolve primary or main shaft at the same speed without any charge in the torque the main shaft is driven through a dog clutch in this gear.

Reverse: In this gear, the peed reduction is usually same as that in the first gear. But the direction of rotation of the main shaft will be reversed by introducing an idler in it. It is due to this change in the direction of rotation of the driving wheels provided by the idler that the motor vehicle moves in reverse direction.